1. Field of the Invention
This invention relates to photographic silver halide spectrally sensitized by means of at least one heat-bleachable spectral sensitizing dye comprising a cyanine dye nucleus and a particular N-methylene substituent. The invention also relates to a method of developing an image in the photographic silver halide and heat bleaching, especially at elevated pH, the spectral sensitizing dye.
2. Description of the State of the Art
Spectrally sensitized photographic silver halides in photothermographic materials and processes are well known, such as described in Research Disclosure, June, 1978, Item No. 17029. Spectral sensitizing dyes in such photothermographic materials can cause the materials to be colored. The resulting color remains in the processed material because the spectral sensitizing dyes are not removed from the photothermographic material during processing. The color from the spectral sensitizing dye imparts an objectionable background to the developed image in the photothermographic material. The problem caused by the presence of colored spectral sensitizing dyes is emphasized in photothermographic materials designed for dye chemical development because such photothermographic materials have higher concentrations of photographic silver halide and spectral sensitizing dyes than other photothermographic materials. It is desirable to be able to remove the color imparted by spectral sensitizing dyes to processed photothermographic materials.
We have found that certain classes of cyanine dyes are heat-bleachable, especially at elevated pH, and hence are decolorized during the heat development of the materials in which they are incorporated. These dyes are effective spectral sensitizers for photothermographic materials and are stable in such materials. While some acutance dyes used in photothermographic silver halide materials, such as those described in U.S. Pat. No. 4,197,131, are heat bleachable, they do not spectrally sensitize silver halide in such materials.
The term material herein, such as in photographic material, means photographic element and composition. For example, photothermographic materials herein mean photothermographic elements and photothermographic compositions.